The present invention relates to discharge lamp employed as an artificial light source in an electronic flash device incorporated in a photographic camera, and the electronic flash device mounted to a photographic camera. More particularly, the present invention relates to a discharge lamp emitting stable light by stabilizing a discharge current and an emitting waveform, and the electronic flash device using the same lamp.
FIG. 8 is a cross section of a conventional discharge lamp.
In FIG. 8, main electrodes 118 and 121 are sealed at both the ends of glass bulb 117. Trigger electrode 122 made of transparent and conductive coating is provided on the entire outer surface of bulb 117. Bulb 117 contains a necessary amount of rare gas such as xenon. Main electrode 118 comprises metallic member 119 and sintered metal member 120 mounted to the tip of metallic member 119. Metallic member 119 is made of tungsten, Kovar or the like. Sintered metal member 120 is made by sintering tungsten powder, tantalum powder, or mixed powder of tungsten and tantalum.
The conventional discharge lamp structured above is built in, e.g. an electronic flash device as shown in FIG. 9. FIG. 9 illustrates an automatic electronic flash device which automatically controls the amount of light emitted from the discharge lamp by sensing the light radiated to a photographic object. Power source 123 supplies a high voltage (approx. 300V), and charges main-discharging capacitor 124 with a charging current, thus approx. 300V is applied across capacitor 124. Trigger circuit 125 produces a high trigger voltage to energize discharge lamp 126. Light-emitting control section 127 stops discharge lamp 126 to emit the light on its way. Photo-receptor 128 comprises photo-receiving element 129 and circuit 130 producing a light-emitting-stopping signal.
An operation of the conventional automatic electronic flash device structured above is described hereinafter.
Capacitor 124 is charged at a high voltage with the charging current from power source 123. Trigger circuit 125 is activated to apply a high voltage to a trigger electrode of discharge lamp 126. Then discharge lamp 126 is energized to emit light by charged energy stored in capacitor 124, thereby radiating a photographic object. The light reflected from the object enters photo-receiving element 129. When the amount of light entering to photo-receiving element 129 reaches a given amount, circuit 130 outputs a light-emitting-stopping signal to light-emitting control section 127. Section 127 then conducts switching operation thereby stopping the discharge lamp 126 to emit the light.
FIG. 10 shows waveforms of discharge-current of the conventional discharge lamp. FIG. 11 shows waveforms of the light emitted from the same bulb. Discharge lamp 126 is energized with a trigger voltage produced by trigger circuit 125, and lamp 126 is discharged by the energy charged in capacitor 124. The waveforms in FIG. 10 illustrate time-varient discharge current. This discharge-current rises sharply approx. at the same time when the trigger voltage is applied, and then starts flowing. When light-emitting-stopping section 127 does not operate switching on the way of emitting the light, i.e. when section 127 is in a complete emitting mode, this flash device finishes discharging by consuming all energy charged in capacity 124 toward an photographic object away from the camera.
On the other hand, brightness of discharge lamp 126 starts increasing not simultaneously with the start of flowing the discharge current but with some time lag, as shown in FIG. 11. When the conventional discharge lamp shown in FIG. 8 is employed, the discharge current of this bulb 126 draws different waveforms marked with 200 and 250 in FIG. 10 at each firing of the lamp, and no stable waveforms are obtained. Light emission of lamp 126 also draws different waveforms marked with 300 and 350 in FIG. 11 at each firing, and no stable waveforms are obtained. In particular, the unstable light emission waveforms as shown in FIG. 11 cause a reduction in precision in automatic light emission control.
It is necessary to detect precisely an amount of reflective lightxe2x80x94out of the light emitted from discharge lamp 126xe2x80x94from a photographic object for realizing precise control over the light emission from lamp 126. For that purpose, photo receptor 128 should synchronizes exactly with an emission timing of lamp 126. There are two methods for activating photo receptor 128; (1) Trigger circuit 125 energizes discharge lamp 126, and the discharge current shown in FIG. 10 starts flowing. At the same time, an operable voltage is supplied to photo-receptor 128. (2) When the discharge current reaches a given amount, this is detected and then the operable voltage is supplied to photo-receptor 128.
When method (2) is employed for powering photo-receptor 128, light-emitting waveform varies every time the discharge lamp fires as shown in FIG. 11 and this causes the following inconvenience: Although photo receptor 128 is ready to detect reflective light from the object, if lamp 126 would delay emitting as shown with waveform 350, receptor 128 receives external lights other than the reflective light from the object during this delay, i.e. a period before lamp 126 starts emitting. Then, receptor 128 cannot receive the reflective light exactly from the object, and thus the light amount radiated to the object is less than an appropriate amount.
On the contrary, when photo-receptor 128 starts operating later than discharge lamp 126, e.g. as shown with waveform 300 in FIG. 11, lamp 126 have already started emission before receptor 128 becomes ready to receive the reflective light from the object. The reflective light is thus not received by receptor 128 until receptor 128 is ready, and thus the light amount radiated to the object exceeds the appropriate amount.
The discussion described above proves that a slight time lag between a light emission timing and an operation start timing of the photo-receptor affects the amount of light emission only a little when the object is away from the camera. However, it affects the amount of light emission substantially when the object is close to the camera.
Independent of the prior art discussed above, Japanese Patent Application Non-Examined Publication No. S57-165948 discloses a flash discharge lamp of which noise at turning on to the peripheral systems is reduced. In this lamp, electrodes of anode and cathode are disposed closely to a line trigger electrode provided along the outer wall of a glass tube. This arrangement allows an instantaneous voltage drop in a waveform of a trigger signal to be reduced, thereby lowering the noise. The electrodes of anode and cathode are closely disposed to the line trigger electrode, so that discharge between the anode and cathode occurs along the trigger electrode. This may somewhat contribute to stabilizing discharge current and discharged light emitting comparing with the method previously discussed; however, this method still does not produce a satisfactory result because of the following reason: Indeed, the section, where the electrodes of anode and cathode are closely placed to trigger electrode 4, forms an acute angle comparing with the center section of the glass tube; however, the anode and cathode face each other in parallel, so that the discharged current and the waveform of light emitted are not always stabilized while the waveform of a trigger signal is stabilized due to the acute angle formed by the anode and cathode is closely placed to the trigger electrode.
The present invention addresses the problem discussed above and aims to provide a discharge lamp emitting light with stable waveforms of both discharge-current and light-emission. This discharge lamp is employed in an electronic flash device which emits the light by consuming the energy charged in a main capacitor, so that the electronic flash device emitting stable light is obtainable.
The discharge lamp of the present invention comprises the following elements:
(a) a glass bulb;
(b) a pair of main electrodes sealed in at both ends of the bulb;
(c) a trigger electrode provided on outer surface of the glass bulb in part in circumference direction and in a longitudinal direction of the bulb; and
(d) rare gas sealed in the bulb.
At least one of the main electrodes includes a metallic member sealed at a first end of the bulb and sintered metal member disposed in the bulb and mounted to this metallic member. The sintered metal member slopes with respect to another main electrode opposite thereto and the tip of the slope is positioned within a limited space covered by the trigger electrode.
This structure allows the discharge lamp to produce constantly stable waveforms of both the discharge current and light emission.
An electronic flash device of the present invention comprises the following elements:
(a) a power source;
(b) a main discharging capacitor to be charged by the power source;
(c) a trigger circuit; and
(d) the discharge lamp discussed above having a trigger electrode on an outer surface of a glass bulbxe2x80x94the trigger circuit applying a voltage to the trigger electrodexe2x80x94for emitting light by consuming energy charged in the main discharging capacitor.
The discharge lamp of the present invention is used in the electronic flash device discussed above, so that the device produces constantly stable waveform of light emission. When the discharge lamp is employed in an automatic electronic flash device that controls light emission automatically, precise control over light emission can be expected. The automatic light emission control stops the lamp to emit the light when received amount of reflective lightxe2x80x94out of emitted light of the discharge lampxe2x80x94from a photographic object reaches an appropriate amount.